1. Field of the Invention
The present invention relates to electronic equipment having a key input function such as an electronic portable calculator (to be referred to as an electronic calculator for brevity hereinafter).
2. Description of the Prior Art
The internal structure of a conventional electronic calculator is shown in FIG. 1. An upper case 1 and a lower case 2 of a housing are both made of a plastic material such as ABS resin. A decorative laminated sheet 3 of an aluminum thin layer is formed on the upper surface of the upper case 1 as needed. A plurality of key tops 4 are arranged at equal intervals at a key input section and are slidably fitted in the holes formed in the upper case 1. A key rubber sheet is disposed at the lower surface of the upper case 1. The key rubber sheet 5 has projections 5a at the positions corresponding to the respective key tops 4. These projections 5a serve to provide upward pressure on the corresponding key tops 4. The key rubber sheet 5 is made of a rubber such as silicone rubber, which has relatively stable properties and satisfactory compressive recovery force. The shape and function of the key rubber sheet 5 will further be described. Each projection 5a urging the lower surface of the corresponding key top 4 upward has an electrically conductive rubber member 5b containing an electrically conductive material such as carbon on its inner, lower surface. The electrically conductive rubber member 5b is obtained by punching an electrically conductive rubber sheet into a predetermined shape. The electrically conductive rubber members 5b are inserted at predetermined positions in a mold before vulcanization and the key rubber sheet 5 is then vulcanized and molded. Thus, the manufacture of the key rubber sheet 5 requires, before fixing of the electrically conductive members 5b, steps of pressing an electrically conductive rubber, insertion of the electrically conductive rubber members 5b in the mold, and so on. This results in a higher manufacturing cost.
An inclined thin skirt 5c surrounds each projection 5a of the key rubber sheet 5. The bottom of the skirt 5c terminates in a base 5b. The skirts 5c formed integrally with each other by the bases 5d and the projections 5a connected to the skirts 5c are arranged immediately below the corresponding key tops 4. Thus, the skirts 5c and the projections 5a number the same as the key tops 4. A hard printed circuit board 6 made of paper phenol, a glass epoxy or the like is fixed to the lower surfaces of the bases 5b. The hard printed circuit board 6 is fixed to the upper case 1 with tapping screws. The base 5d of the key rubber sheet 5 is sandwiched and fixed between the hard printed circuit board 6 and the upper case 1. When the upper surface of a key top 4 is depressed with a finger or the like in the key input section as described above, the projection 5a of the key rubber member 5 is urged and the skirt 5c is gradually deformed under pressure to increase its resistance. However, when the key top 4 is further depressed, the skirt 5c is bent and abruptly looses its resistance to the downward force. Then, a clicking is felt by the finger, and the lower surface of the electrically conductive rubber member 5b contacts the upper surface of the hard printed circuit board 6. A key input contact pattern formed in a predetermined shape is bridge-connected by the conductive rubber member 5b, and a key input signal is supplied to an LSI 7 as an arithmetic/processing section of the electronic calculator. The LSI 7 processes the key input signal and produces a display signal. The display signal is supplied to liquid crystal display elements 9 through a circuit pattern on the hard printed circuit board 6 and urging interconnectors 8. The liquid crystal display elements 9 display characters, symbols and the like. In general, metallic pins 7a of the LSI 7 are soldered to the circuit pattern of the hard printed circuit board 6. Therefore, a relatively expensive printed circuit board which is capable of withstanding soldering heat is used. However, in the case of a printed circuit board which extends up to the key input section where there are no parts to be soldered as in the case of this conventional electronic calculator, the area of the printed circuit board 6 is increased. Thus, only a smaller number of printed circuit boards may be obtained from a printed circuit board sheet of a predetermined area, resulting in higher cost of the electronic calculator.